Optical devices are used in a wide variety of applications ranging from telecommunications to medical technology.
It is desirable to hermetically seal optical devices to prevent deterioration in performance due to moisture and other species present in the atmosphere. Furthermore, it is desirable to improve the reliability of hermetically sealed optical devices.
A perennial problem has been that stresses originating from thermal expansion, thermal gradients, the device's mounting process, or other causes can cause seal failure or even complete or partial detachment of hermetically sealed components in an optical device.
In hermetically sealing a lid to a package using a soldering process, it is common practice to use an overlap joint, such as shown in FIGS. 1a and 1b, or a variation thereof. FIG. 1a shows a schematic presentation of a prior art hermetically sealed device wherein an overlap joint is provided by sealing a box 110 with a solder 120 to a lid 130. FIG. 1b is a close-up view of Section A of FIG. 1a showing in more detail the overlap joint between planar surfaces of box 110 and lid 130 by means of solder 120. Furthermore, it is noted, that with such overlap joints, it is possible to achieve compressive stresses in the plane of the joint but not in the lateral direction. The presence of compressive stress in the lateral direction of the joint will improve the toughness of the joint and thereby enhance its reliability.
Consequently, since such prior art soldering techniques between a lid and a package make use of planar surfaces, the stress forces lie in a two-dimensional plane. Therefore, there is a need for hermetically sealed devices wherein the stress forces act in three dimensions. The advantages of such designs are increased reliability and a greater range of materials that can be used in making the solder joint.
It is an object of this invention to provide a hermetic seal with improved reliability.
Another object of this invention is to provide a hermetic seal comprising a three-dimensional state of compressive stresses.
A further object of the invention is to provide a method of making such an improved hermetic seal.